Optical spectroscopy entails passing optical radiation through a sample, often referred to an analyte, and inferring properties of the analyte from measurements performed on the optical radiation. For example, trace gas detection can be spectroscopically performed by performing measurements to detect the presence or absence of spectral absorption lines corresponding to the gas species of interest. Spectroscopy has been intensively developed over a period of many decades, and various ideas have been developed to improve performance.
One such idea can be referred to as cavity-enhanced spectroscopy, in which the analyte is disposed within an optical cavity (i.e., an optical resonator). The cavity can enhance the interaction between the analyte and the optical radiation, thereby improving spectroscopic performance. For example, in cavity ring-down spectroscopy (CRDS), the absorption is measured by way of its effect on the energy decay time of an optical cavity. Increased absorption decreases the decay time, and vice versa. As another example, cavity enhanced absorption spectroscopy (CEAS) entails the use of an optical cavity to increase the sensitivity of absorption spectroscopy, in connection with direct absorption measurements.
One of the noteworthy features of cavity-enhanced spectroscopy is that issues of optical alignment can arise which differ in important respects from alignment issues in other branches of optics. More specifically, a key alignment issue faced in many implementations of cavity enhanced spectroscopy is selectively exciting the lowest order mode of a passive optical cavity with an external optical source while minimizing excitation of the higher order modes of the cavity. The theoretical condition for providing such selective mode excitation is well known in the art, and is often referred to as “mode matching”. For example, suppose radiation in the lowest order mode of an optical cavity would be emitted from the cavity as a Gaussian beam having certain parameters (e.g., waist size w0, waist position z0) along a beam axis L. In this example, radiation provided to the cavity as a Gaussian beam with waist size w0 and waist position z0 along beam axis L is mode matched to the lowest order mode of the resonator, and will selectively excite the lowest order mode of the cavity.
Although the theoretical condition for mode matching is well known, practical issues such as assembly tolerances and optical component tolerances can cause substantial difficulties. In this context, it is important to note that the passive cavity alignment problem is a much less forgiving single-mode alignment problem than the extensively explored problem of coupling to a single mode optical fiber or waveguide. The reason for this difference can be appreciated with a simple example where practical imperfections are assumed to cause a 1% loss of power coupled to the desired mode.
In the case of fiber or waveguide coupling, this 1% of the incident light that does not couple to the desired mode is lost from the system. There is typically no degradation of performance other than the 1% loss. In the case of coupling to a passive spectroscopic cavity, the 1% of the incident light that does not couple to the desired lowest order mode can couple to one or more of the higher order modes of the cavity. Such excitation of undesired cavity modes can seriously degrade spectroscopic performance, by effectively raising the noise floor. Such an effective increase in noise is typically a much more significant performance degradation than the 1% signal loss entailed by the above assumption.
Although the importance of achieving the mode matching condition is well known (e.g., as indicated in U.S. Pat. No. 5,912,790), specific methods for providing mode matching to a passive cavity in practice do not appear to have been explicitly considered in the art. US 2005/0168826 is an example where a somewhat related alignment problem is considered. In this work, an alignment system including a weak lens provides coupling of a source to a single mode waveguide. Coupling efficiency to the waveguide is enhanced by adjusting the position and angles of the weak lens during assembly. Another somewhat related problem of alignment has been considered in U.S. Pat. No. 6,563,583, where alignment is required to a multi-pass cell as opposed to an optical cavity. In this work, active feedback control is employed to measure and correct beam position and angle errors.
However, it is preferable to provide the level of alignment precision needed for cavity enhanced spectroscopy with an optical system having no moving parts, to reduce cost and simplify the resulting system. Accordingly, it would be an advance in the art to provide improved mode matching to a passive optical cavity while allowing for fabrication and assembly tolerances.